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I HC QUC GIA TP.H CH MINHTRNG I HC BCH KHOA

KHOAIN-IN TB MN K THUT IN T

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Embedded System Design

Chapter 3: C Programming for PICMicrocontroller

B mn K Thut in T - HBK

References

Textbook

Martin Bates, Programming 8-bit PIC Microcontrollers inC, Newnes, 2008

Many C compilers for PIC:MikroC (www.mikroe.com)

PICC18 (www.htsoft.com)

MPLAB C18, C30 (www.microchip.com)

CCS C (www.microchipc.com/reviews/CCS_C/)

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Outline

2.1 PIC16 C Getting Started

Simple program and test circuit

Variables, looping, and decisions

SIREN program

2.2 PIC16 C Program Basics

Variables

Looping

Decisions

2.3 PIC16 C Data Operations

Variable types

Floating point numbers

Characters

Assignment operators

2.4 PIC16 C Sequence Control

While loops Break, continue, goto If, else, switch

2.5 PIC16 C Functions and Structure

Program structure Functions, arguments Global and local variables

2.6 PIC16 C Input and Output

RS232 serial data Serial LCD Calculator and keypad

2.7 PIC16 C More Data Types

Arrays and strings Pointers and indirect addressing Enumeration

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2.1 PIC16 C Getting Started

Microcontroller programs contain three main features:

Sequences of instructions Conditional repetition of sequences Selection of alternative sequences

Listing 2.1 A program to output a binary code

/* Source code file: OUTNUM.C

Author, date, version: MPB 11-7-07 V1.0

Program function: Outputs an 8-bit code

Simulation circuit: OUTBYTE.DSN

*******************************************************/

#include "16F877A.h" // MCU select

void main() // Main block

{

output_D(255); // Switch on outputs

}

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Figure 2.1 MPLAB IDE Screenshot

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Figure 2.2 ISIS dialogue to attach program

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Setup IO Ports Port modes

#use fast_io(port): leaves the state of the port the same unless re-configured

#use fixed_io(port_outputs=pin, pin): permanently sets up the datadirection register for the port

#use standard_io(port): default for configuring the port every time its used

Set directions set_tris_a(value);

value = get_tris_a();

Read / write IO ports value = input_A();

output_A(value);

output_high(pin); //set an output to logic 1

output_low(pin); //set an output to logic 0

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PIC16 C Program Basics Variables Looping Decisions

The purpose of an embedded program is to read in data or control inputs, to process them and operate the outputs as required.

The program for processing the data usually contains repetitive loopsand conditional branching, which depends on an input or calculatedvalue.

2.2 PIC16 C Program Basics

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Variables

Variables: is a label attached to the memory location where the variable value is

stored.

automatically assigned to the next available location or locations (manyvariable types need more than 1 byte of memory).

must be declared at the start of the program block, so that the compilercan allocate a corresponding set of locations.

Only alphanumeric characters (az, AZ, 09) can be used for variablenames

Variable values

in decimal by default in hexadecimal with the prefix 0x, for example, 0xFF

By default, the CCS compiler is not case sensitive,

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Listing 2.2 Variables

/* Source code file: VARI.C


Program function: Outputs an 8-bit variable


*******************************************************/

#include "16F877A.h"

void main()

{

int x; // Declare variable and type

x=99; // Assign variable value

output_D(x); // Display the value in binary

}

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Looping Most real-time applications need to execute continuously until the

processor is turned off or reset.

In C this can be implemented as a while loop, as in Listing 2.3 .

/* Source code file: ENDLESS.C


Program function: Outputs variable count


*******************************************************/

#include "16F877A.h

void main()

{

int x; // Declare variable

while(1) // Loop endlessly

{ output_D(x); // Display value

x++; // Increment value

}

}

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Decision Making

The effect of the programis to switch on the output ifthe input is high.The switch needs to beclosed before running tosee this effect.The LED cannot beswitched off again until theprogram is restarted.

The simplest way to illustrate basic decision making is to change an outputdepending on the state of an input.

Figure 2.4 show test circuit with input switch

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Listing 2.4 IF statement

/* Source code file: IFIN.C


Program function: Tests an input

Simulation circuit: INBIT.DSN

*******************************************************/


void main()

{

int x; // Declare test var.

output_D(0); // Clear all outputs

while(1) // Loop always

{x = input(PIN_C0); // Get input

if(x==1)output_high(PIN_D0); // Change out

}

}

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Loop Control

The program can be simplified by combining the input functionwith the condition statement as follows:

if (input(PIN_C0)) output_high(PIN_D0);

The conditional sequence can also be selected by a whilecondition.

In Program WHILOOP.C ( Listing 2.5 ) the input is tested in the loop condition statement and the output flashed on

and off while the switch is open (input high).

If the switch is closed, the flash loop is not executed and the LED is switched off.

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Listing 2.5 Conditional loop

/* Source code file: WHILOOP.C


Program function: Input controls output loop


*******************************************************/


#use delay (clock=1000000) // MCU clock = 1MHz

void main(){

while(1)

{

while(input(PIN_C0)); // Repeat while switch open

{ output_high(PIN_D0);

delay_ms(300); // Delay 0.3s

output_low(PIN_D0);delay_ms(500); // Delay 0.5s

}

output_low(PIN_D0); // Switch off LED

}

}

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FOR Loop

The WHILE loop repeats until some external event or internallymodified value satisfies the test condition.

In other cases, we need a loop to repeat a fixed number oftimes.

The FOR loop uses a loop control variable, which is set to aninitial value and modified for each iteration while a defined

condition is true.

In the demo program FORLOOP.C ( Listing 2.6 ), the loop controlparameters are given within the parentheses that follow the for

keyword.

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FOR Loop

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SIREN Program

A program combining some of these basic features is shown inSIREN.C ( Listing 2.7 ).

This program outputs to a sounder rather than an LED,operating at a higher frequency.

The output is generated when the switch is closed (input C0low).

The delay picks up the incrementing value of step giving alonger pulse each time the for loop is executed.

This causes a burst of 255 pulses of increasing length (reducingfrequency), repeating while the input is on.

Note that 255 is the maximum value allowed for step, as it isan 8-bit variable.

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Listing 2.7 Siren Program

/* Source code file: SIREN.C


Program function: Outputs a siren sound


*******************************************************/


#use delay (clock=1000000)

void main()

{

int step;

while(1)

{

while(!input(PIN_C0)) // loop while switch ON

{

for(step=0;step

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Blank Program

A blank program is shown in Listing 2.8 , which could be usedas a general template.

We should try to be consistent in the header commentinformation, so a standard comment block is suggested.

Compiler directives are preceded by hash marks and placedbefore the main block.

Other initialization statements should precede the start of themain control loop. Inclusion of the unconditional loop option

while(1) assumes that the system will run continuously until

reset.

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Table 2.1 A basic set of CCS C components

Compiler Directives#include source files Include another source code or header file#use functions(parameters) Include library functions

C Blocks

main(condition) {statements } Main program block

while(condition) {statements } Conditional loopif(condition) {statements } Conditional sequencefor(condition) {statements } Preset loop

C Functions

delay_ms(nnn) Delay in millisecondsdelay_us(nnn) Delay in microsecondsoutput_x(n) Output 8-bit code at Port Xoutput_high(PIN_nn) Set output bit highoutput_low(PIN_nn) Set output bit lowinput(PIN_nn) Get input

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Variable types Floating point numbers Characters Assignment operators

A main function of any computer program is to carry out calculationsand other forms of data processing. Data structures are made up ofdifferent types of numerical and character variables, and a range ofarithmetical and logical operations are needed.

Microcontroller programs do not generally need to process largevolumes of data, but processing speed is often important.

2.3 PIC16 C Data Operations

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Table 2.1 Integer Variables

Name Type Min Max

int1 1 bit 0 1

unsigned int8 8 bits 0 255

signed int8 8 bits -127 +127


signed int16 16 bits -32767 +32767


signed int32 32 bits -2147483647 +2147483647

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Table 2.2 Microchip/CCS Floating Point Number Format

Exponent Sign Mantissa

xxxx xxxx x xxx xxxx xxxx xxxx xxxx xxxx

8 bits 1 23 bits

Table 2.4 Example of 32-bit floating point number conversion

Mantissa: 101 0010 0000 0000 0000

0000

Exponent: 1000 0011

Sign: 1 = negative number

FP number: 1000 0011 1101 0010 0000 0000 0000

0000

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Figure 2.5 Variable Types

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Figure 2.6 Variable Operations

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Table 2.7 Conditional Operators

Operation Symbol EXAMPLE

Equal to == if(a == 0) b=b+5;

Not equal to != if(a != 1) b=b+4;

Greater than > if(a > 2) b=b+3;

Less than < if(a < 3) b=b+2;

Greater than or equal to >= if(a >= 4) b=b+1;

Less than or equal to

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2.4 PIC16 C Sequence Control

While loops

Break, continue, goto

If, else, switch

Conditional branching operations are a basic feature of anyprogram.

These must be properly organized so that the programstructure is maintained and confusion avoided.

The program then is easy to understand and more readilymodified and upgraded.

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While Loops

The basic while(condition) provides a logical test at the start ofa loop, and the statement block is executed only if the

condition is true.

It may, however, be desirable that the loop block be executedat least once, particularly if the test condition is affected

within the loop.

This option is provided by the do..while(condition) syntax. Thedifference between these alternatives is illustrated in Figure

2.7 .

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ConditionTrue?

StatementBlock Condition

True?

StatementBlock

(a) While loop (b) Do..While loop

Figure 2.3.1 Comparison of While and Do..While Loop

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Listing 2.9 DOWHILE.C contains both types ofwhile loop

// DOWHILE.C

// Comparison of WHILE and DO WHILE loops

#include "16F877A.H

main()

{

int outbyte1=0;

int outbyte2=0;

int count;

count=0; // This loop is notwhile (count!=0) // executed

{ output_C(outbyte1);

outbyte1++;

count--;

}

count=0; // This loop is

do // executed

{ output_C(outbyte2);

outbyte2++;

count--;

} while (count!=0);

while(1){};

}

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Break, Continue, and Goto

Break and continue It may sometimes be necessary to breakthe execution of a loop or

block in the middle of its sequence.

The block must be exited in an orderly way, and it is useful to have theoption of restarting the block (continue) or proceeding to the next one

(break).

Goto Occasionally, an unconditional jump may be needed, but this should

be regarded as a last resort, as it tends to threaten the program

stability.

It is achieved by assigning a label to the jump destination andexecuting a goto..label.

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label

StatementBlock

ContinueGoto

Break

Figure 2.8 Break, continue and goto

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Listing 2.10 Continue, Break & Goto

// CONTINUE.C

// Continue, break and goto jumps

#include "16F877A.H"

#use delay(clock=4000000)

main()

{

int outbyte;

again: outbyte=0; // Goto destination

while(1)

{

output_C(outbyte); // Loop operation

delay_ms(10);

outbyte++;

if (!input(PIN_D0)) continue; // Restart loopif (!input(PIN_D1)) break; // Terminate loop

delay_ms(100);

if (outbyte==100) goto again; // Unconditional jump

}

}

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Figure 2.9 Comparison of If and If..Else

Ifblock

ConditionTrue?

YES

NO

ConditionTrue?

Ifblock

Elseblock

YES NO

If..Else and Switch..Case

If .. Else: allows a block to be executed or skipped conditionally.

Switch..case : When we need a multi-choice selection

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Test Variable

Value = 3? Procedure 3YES

NO

Value = n?Procedure nYES

NO

DefaultProcedure


NO


NO

Figure 2.10 Switch..case branching structure

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Listing 2.11 Comparison of Switch and If..Else control

// SWITCH.C

// Switch and if..else sequence control

// Same result from both sequences

#include "16F877A.h

void main()

{

int8 inbits;

while(1) {inbits = input_D(); // Read input byte

// Switch..case option........................

switch(inbits) // Test input byte

{

case 1: output_C(1); // Input = 0x01,output = 0x01

break; // Quit block





default:output_C(0); // If none of these,output= 0x00

}

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2.5 PIC16 C Functions and Structures

Program structure

Functions, arguments

Global and local variables

The structure of a C program is created using functions. This is a block of code written

and executed as a self-contained process, receiving the required parameters (data

to be processed) from the calling function and returning results to it. Main() is the

primary function in all C programs, within which the rest of the program is

constructed.

When running on a PC, main() is called by the operating system, and control is

returned to the OS when the C program is terminated. In the microcontroller,

main() is simply used to indicate the start of the main control sequence, and more

care needs to be taken in terminating the program.Normally, the program runs in a continuous loop, but if not, the final statement should

be while(1);, which causes the program to wait and prevents the program running

into undefined locations following the application code.

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Figure 2.11 Hierarchical C program structure

Main(){

statementsfun1()statementsstatements................statementsfun2(arg)statements

}

void fun1(){

statements......

}

void fun2(arg){

statements...

fun3...return(val)

}

void fun3{

statements......

}

LEVEL 0 LEVEL 1 LEVEL 2

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Basic Functions

A simple program using a function is shown in FUNC1.C, Listing 2.12 . Themain block is very short, consisting of the function call out() and a while

statement, which provides the wait state at the end of main(). In this case,

the variables are declared before the main block. This makes them global

in scope; that is, they are recognizedthroughout the whole program and

within all function blocks. The function out() is also defined before main() ,

so that, when it is called, the function name is recognized. The function

starts with the keyword void , which indicates that no value is returned by

the function. The significance of this is explained shortly.

The function itself simply increments Port C from 0 to 255. It contains a forloop to provide a delay, so that the output count is visible. This is a simple

alternative to the built-in delay functions seen in previous examples and isused here to avoid the inclusion of such functions while we study user-

defined functions. It simply counts up to a preset

value to waste time. The delay time is controlled by this set value.

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Listing 2.12 Basic function call

// FUNC1.C

// Function call structure

#include "16F877A.H

int8 outbyte=1;

int16 n;

void out() // Start of function block

{

while (outbyte!=0) // Start loop, quit when output =0{

output_C(outbyte); // Output code 1 0xFF

outbyte++; // Increment output

for(n=1;n

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Listing 2.14 Local variables

// FUNC3.C// Use of local variables

#include "16F877A.H

int8 outbyte=1; // Declare global variables

int16 count;

int out(int16 t) // Declare argument types

{

int16 n; // Declare local variable

while (input(PIN_D0)) // Run output at speed t

{ outbyte++;

for(n=1;n

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Serial LCD

CCS C provides an RS232 driver routine that works with anyI/O pin (that is, the hardware port need not be used).

This is possible because the process for generating the RS232data frame is not too complex and can be completed fast

enough to generate the signal in real time.

At the standard rate of 9600 baud, each bit is about 100 slong, giving an overall frame time of about 1 ms.

The data can be an 8-bit integer or, more often, a 7-bit ASCIIcharacter code.

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Serial LCD

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Serial LCD

Control Codes

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Serial LCD Format Codes

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Example

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2.7 PIC16 C More Data Types

Arrays and strings

Pointers and indirect addressing

Enumeration

The data in a C program may be most conveniently handled assets of associated variables. These occur more frequently as

the program data becomes more complex, but only the basics

are mentioned here.

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2.8 PIC16 C Compiler Directives

Include and use directives

Header file listing and directives

Compiler directives are typically used at the top of the program to set up

compiler options, control project components, define constant labels, and

so on before the main program is created. They are preceded by the hashsymbol to distinguish them from other types of statements and do not

have a semicolon to end the line.

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